Oxidation of reactive organic gases (ROG) in ambient air produces low-volatility compounds that may condense to form secondary organic aerosols (SOA). Reactive oxygen species (ROS) are also produced in the photochemical reactions involving ROG In the urban atmosphere, the precursors of SOA and ROS primarily come from vehicular emissions. This study aims at investigating the SOA and ROS formation potentials of diluted motorcycle exhaust under UV irradiation. Exhaust samples were obtained from a 124-cc motorcycle engine under idling conditions. Emissions from the tailpipe were directly collected in a Tedlar bag and diluted with filtered air by a ratio of 5. A filter cassette placed upstream of the bag removed particles in the exhaust while the sample was being collected. The sample in the bag was then irradiated by UV light and the number size distribution of aerosol particles was measured at various time intervals using a scanning mobility particle sizer. The concentrations of gaseous and particulate reactive oxygen species were also determined at intervals. Under continuous UV irradiation, the particle number concentration in the bag peaked at 2-3 h, while the gaseous ROS concentration peaked at 5-7 h. The peak concentration of gaseous ROS was about one order of magnitude higher than the initial value in the diluted motorcycle exhaust. From the difference between gaseous and total ROS concentrations, it was estimated that about 9.6% (by mole number) of the total ROS was in the particulate phase. The results suggest that diluted motorcycle exhaust has a high potential to form ROS and SOA under UV irradiation.